1. Field of the Invention
The present invention relates to via placement and routing, and more particularly to techniques for via placement and routing that optimize electrical performance and layout efficiency.
2. Description of Related Art
An integrated circuit (IC) is a fragile piece of semiconductor that is easily damaged. Therefore, an IC is typically placed inside a package designed to protect the IC from damage that can occur during handling. An IC package is also designed to connect an IC to a printed circuit board (PCB) or other type of circuit board.
The pin connections on a PCB are usually much more widely spaced than the pins on an integrated circuit die. For this reason, a PCB cannot be directly connected to an integrated circuit die. Because the pins on an IC are densely packed together, a package spreads out signal wires routed from the IC pins so that they can be coupled to more widely spaced connections on a PCB. Thus, a package is a connector that provides an intermediate connectivity layer between an IC die and a circuit board.
A package typically contains a set of dielectric layers (including a core layer) and a set of conductive layers. Vias (i.e., holes) are bored through the layers of a package and filled with a conductive material. Signals such as a power supply voltage (VDD), ground, input signals, and output signals are routed through the vias between the IC pins and the PCB connections.
The vias in a package that route the supply voltage connections to an IC are part of a power distribution network (also called a power network). The vias in a package that route the ground connections to an IC are part of a ground network. The vias in a package that route the input and output signals to and from the IC are part of an IO network.
Vias are typically placed individually within a package without regard to the relationships between networks. Ground vias are often placed, for example, at a 1-2 mm pitch opportunistically without any regard to their relationship to the IO network. Power supply vias are not routed in any relationship with respect to ground vias.
An IO network in a package often suffers from cross-talk. Cross-talk is interference that is caused by mutual capacitance and/or mutual inductive coupling between two adjacent IO vias (or wires). Cross-talk can cause the transmitted information to become distorted, possibly causing a partial or total loss of data. Cross-talk becomes more significant as the spacing between IO vias is reduced. One way to reduce cross-talk in a package is to decrease the thickness of the core layer to cut back on the mutual inductance.
A power network in a package often contains a significant amount of impedance. Impedance can cause problems in a power network. Impedance in a package causes a voltage drop in the power supply that can adversely affect circuit performance. For example, a significant voltage drop in the power supply voltage can effect circuit timing and/or slow down circuit operation in the IC significantly.
Therefore, it would be desirable to provide techniques for reducing cross-talk and impedance in vias that route signals through connectors.